The present invention relates to a measurement method and apparatus.
A conventional projection exposure apparatus projects a circuit pattern of a reticle onto a photosensitive substrate, such as a wafer, via a projection optical system in manufacturing fine semiconductor devices, such as a semiconductor memory and a logic circuit, using the photolithography technology.
Since a projection exposure apparatus is required to precisely transfer a reticle pattern onto a wafer at a predetermined magnification or reduction ratio, use of a projection optical system having an extremely restrained aberration or a superior imaging characteristic is important. In particular, along with the increasing demand for fine processing to semiconductor devices, a transfer pattern is sensitive to the aberration of an optical system. Therefore, there is a demand for a precise measurement of a optical characteristic, such as a wavefront aberration, of the projection optical system. A simple, prompt, and inexpensive measurement is important to improve the mass production and the economical efficiency.
A point diffraction interferometer (“PDI”) that has a pinhole that forms an ideal spherical wave, and a lateral shearing interferometer (“LSI”) that utilizes the shearing interferometry have conventionally been known as a measurement apparatus that measures a wavefront aberration of the optical system. Recently, a measurement apparatus that utilizes a linear diffraction interferometer (“LDI”) having a slit that forms an ideal cylindrical or elliptical wave has been known. See Japanese Patent Applications, Publication Nos. 2000-97666 and 2005-244126.
The LDI type measurement apparatus mounted on the exposure apparatus arranges a plate having a measurement mark on an image plane side of the projection optical system. The measurement mark has a slit that forms an ideal wavefront, and a window that transmits a wavefront of aberrational information of the projection optical system. A two-dimensional light-receiving element is arranged below the plate, and takes an image of interference between two wavefronts generated from the plate. The wavefront aberration of the projection optical system is measured through image processing to the captured interference pattern (interferogram) image.
However, the conventional measurement apparatus cannot precisely measure the wavefront aberration of the projection optical system.
The above image processing contains an image conversion process that converts an interference pattern image from a light receiving plane coordinate to a pupil plane coordinate of the projection optical system, and a wavefront aberration information calculating process that calculates wavefront aberration information from the image-converted interference pattern data.
The image conversion process calculates through ray tracing which coordinate position on the pupil plane in the projection optical system an interference pattern data value at each coordinate position on the light receiving plane comes from. Therefore, it is necessary to know a structure of each member between the pupil plane of the projection optical system and the light receiving plane. A thickness of a plate, a distance between the plate and the light receiving plane, and a positional relationship between the measurement mark and the light receiving plane (a center and radius of the interference pattern on the light receiving plane), which define a structure of the measurement apparatus, are used for input parameters for the image conversion process. A “process parameter” is a generic term of input parameters in this specification.
Among the process parameters, prior art use design values for the thickness of the plate, and the distance between the plate and the light receiving plane. In addition, for a center and radius of the interference pattern, prior art uses a design value, determines it by detecting a region through image processing, or defines it through visual inspections of the pattern region in the interference pattern image. However, a shift occurs between the design value and the actual value due to manufacturing errors of the apparatus, such as a telecentric difference for each image point of the projection optical system, a manufacture error of a plate, and an arrangement error of the light receiving element. When the design value is used for a process parameter, a precise image conversion process cannot be performed, and calculated wavefront aberration information contains a measurement error.
Moreover, even when an image conversion process is performed using for a process parameter a design value that has no shift from an actual value, the wavefront aberration information contains a measurement error, because a manufacturing error of the apparatus cannot be eliminated, such as inclinations among a bottom plane of the projection optical system, a plate plane, and an inclination of a light receiving plane.
This problem is not limited to the LDI type measurement apparatus, but other measurement apparatuses that measure as two-dimensional information an optical characteristic of an optical system, such as a PDI type measurement apparatus and a LSI type measurement apparatus.